A large birefringence and a low control voltage distinguish nematic liquid crystals (LCs) from other electro-optical materials. For a lens based on LC materials, a voltage is employed traditionally to vary its focus. Much of the past work on adaptive nematic LC lenses has focused on the lens structure, the focal length of the lens and the ability to tune the lens. One of the fundamental parameters describing a lens is its focal length. For virtually all lenses, the focal length is a static parameter, which means that, once the lens is constructed, the focal length is set. For a variable focal length to be obtained, usually a group of lenses, having separation distances between the lenses, is adjusted mechanically. Adjustments accomplished with mechanical movement are inherently bulky and inefficient. In comparison to a lens that has only one focal length, the tunable LC lens provides a very important optical element. Acting as a group of lenses, an adaptive LC lens is compact, lightweight, efficient and low cost. Therefore, it is desirable to have continuously tunable, adaptive nematic LC lenses, as disclosed in the present invention.
One type of prior art adaptive nematic LC lens has a non-uniform or inhomogeneous LC layer, as discussed below. Bricot et al. in U.S. Pat. No. 4,037,929 describe a kind of LC lens consisting of a plano element and a convex element between which the LC is stored. Both the inner surfaces of the planar element and the convex element are coated with a transparent electrode. As taught by Bricot et al., the applied voltage can change the LC refractive index. Upon changing the refractive index of the LC material, the focal length of the lens can be tuned within a narrow range because of the inherent surface profile. Due to the non-uniform LC cell gap, LC alignment along the inner convex surface becomes worse; thus, light is scattered by the LC layer.
Berreman in U.S. Pat. No. 4,190,330, Okada et al. in U.S. Pat. No. 4,795,248, Toda et al. in U.S. Pat. No. 5,047,847, and Takahashi et al. in U.S. Pat. No. 5,150,234 each disclose a similar LC lens structure wherein the substrate surface or the LC gap layer presents a convex or concave profile. Okada et al. in U.S. Pat. No. 4,904,063, disclose a Fresnel lens; the inner surface of one substrate is formed by a number of annular ring-shaped Fresnel grooves with their crests leveled for more rapid response time. Kikuchi in U.S. Pat. No. 5,124,836 and Nishioka in U.S. Pat. No. 6,437,925 each disclose a variable focal length lens with the added element of a mirror to compensate for the curved LC layer or substrate surface.
Another type of LC lens is one wherein the LC layer is uniform or homogeneous, but with different electrode patterns. Kowel in U.S. Pat. No. 4,572,616 describes a LC lens with a homogeneous LC layer but with a series of independently linear transparent control electrodes to make the lens tunable. The demerits of this kind of lens are that the operating technique is complicated, and there is light diffraction due to the grating-like electrode. Tajima in U.S. Pat. No. 6,191,881, U.S. Pat. No. 6,512,563 and Hamada et al. in U.S. Pat. No. 6,577,434 describe a lens structure similar to that of Kowel in U.S. Pat. No. 4,572,616, but made with Fresnel zone structures.
The lens device of the present invention is fabricated with planar LC layers and planar substrates, but curved electrodes. The curved electrodes can be convex, concave, spherical or annular ring-shaped grooves, as in a Fresnel lens. LC lens with the structures outlined above can be switched with uniform optical response due to a homogeneous cell gap; no light scattering due to homogeneous alignment, and the focal length of the novel lens device can be tuned from infinity to a desired definite range.